Branched vapor-grown carbon fiber, electrically conductive transparent composition and use thereof

ABSTRACT

A branched vapor-grown carbon fiber having an outer diameter of 0.5 μm or less and an aspect ratio of at least 10, the carbon fiber having a compressed specific resistance of 0.02 Ω·cm or less, each fiber filament having a hollow cylindrical structure, preferably the carbon fiber containing boron and having a compressed specific resistance of 0.018 Ω·cm or less. An electrically conductive transparent composition comprising a resin binder and carbon fiber incorporated into the binder, having transparency and comprising vapor grown carbon fiber having an outer diameter of 0.01–0.1 μm, an aspect ratio of 10–15,000, and a compressed specific resistance of 0.02 Ω·cm or less, and surface resistivity of 10,000 Ω/□ or less. An electrically conductive transparent material formed from the aforementioned electrically conductive transparent composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is an application filed pursuant to Section 111(a) with a claim to priority to Provisional Application Ser. Nos.60/267,176 and 60/267,179 filed Feb. 8, 2001 pursuant to 35 U.S.C.Section 119(e) (1) in accordance with 35 U.S.C. 111(b).

TECHNICAL FIELD

The present invention relates to vapor grown carbon fiber exhibiting anenhanced function when used as an electrically conductive or heatconductive filler for composite materials, such as resin- orrubber-based composite materials or an enhanced function when used as anadditive which may be incorporated into the electrodes of variousbatteries, such as lead storage batteries and to a process for producingit. The present invention also relates to an electrically conductivetransparent composition containing a resin and carbon fiber incorporatedinto the resin, which composition does not lose transparency inherent tothe resin and exhibits both electrical conductivity and transparency.The electrically conductive transparent composition of the presentinvention is useful as an electrically conductive transparent materialin a variety of materials requiring light transmission and electricalconductivity, for example, electrically conductive transparent coating,electrically conductive transparent film, or electrically conductivetransparent sheet.

BACKGROUND ART

In general, electrically conductive coating, film, or sheet is producedfrom a mixture containing electrically conductive material and paint orfilm material. Widely used electrically conductive materials includemetallic powder, electrically conductive inorganic oxide powder, andcarbon powder. However, metallic powder has a drawback in that theelectrical conductivity of the powder is lowered through oxidation orcorrosion. Furthermore, when a noble metal (e.g., silver), which doesnot easily undergo oxidation or corrosion, is used for, for example,wires of an IC, etc., the noble metal involves problems, including shortcircuit due to migration. Although carbon powder does not have such adrawback of metallic powder, the electrical conductivity of carbonpowder is lower than that of metallic powder. Therefore, in order toenhance electrical conductivity, there have been proposed, for example,carbon fiber which is easily graphitized and has a specific structure inwhich an aspect ratio is large (Japanese Patent Publication (kokoku) No.06-39576), or a material containing entangled carbon fiber filaments(Japanese Patent Application Laid-Open (kokai) No. 07-102197).

However, in the case where the aforementioned electrically conductivematerial is incorporated into a resin, a problem arises thattransparency inherent to the resin may be lost when the incorporationamount of the conductive material is increased in order to enhance theelectrical conductivity of the resin. For example, when a materialcontaining entangled carbon fiber filaments is incorporated into aresin, the incorporation amount of the material must be tens of mass %in order to secure sufficient enhancement of the electrical conductivityof the resin. As a result, when the thickness of a coating or a filmformed from the resin is about 1 mm, the transmittance of the coating orfilm becomes about 30%; i.e., the coating or film becomes opaque andbarely transmits light. In contrast, when the amount of carbon fiberincorporated into a resin is reduced in order to maintain thetransparency of the resin, the electrical conductivity of a coating orfilm formed from the resin is greatly reduced.

There has also been proposed an electrically conductive transparentcomposition prepared from an electrically conductive material to which,in order to enhance electrical conductivity, a mixture of graphitehaving an average particle size of 1–20 μm and carbon powder having aBET specific surface area of 25–800 m²/g has been incorporated (JapanesePatent Application Laid-Open (kokai) No. 2000-173347). However, when thecomposition is formed to have a thickness of 0.02–0.5 μm and atransmittance of 30%, the surface resistivity of the composition is1×10⁵ Ω/□ (ohm/square)(or simply referred to Ω, hereinafter the samewill do); i.e., the electrical conductivity of the composition is stilllow. As described above, conventional electrically conductive coating orelectrically conductive film encounters difficulty in attaining bothtransparency and high electrical conductivity.

An object of the present invention is to overcome the aforementionedproblems of conventional electrically conductive coating or electricallyconductive film and to provide an electrically conductive transparentcomposition comprising carbon fiber, in particular vapor grown carbonfiber (hereinafter sometimes abbreviated as “VGCF”), of very small outerdiameter and high electrical conductivity, which composition does notlose transparency inherent to a resin and exhibits both transparency andhigh electrical conductivity; and an electrically conductive transparentmaterial formed from the composition.

Vapor grown carbon fiber (VGCF) is produced by thermally decomposing araw material gas, such as hydrocarbon gas, in a vapor phase in thepresence of a metallic catalyst, and by growing the decompositionproduct into a fibrous shape. It has been known that carbon fiber havinga diameter of tens of nm to 1,000 nm can be produced through thisprocess.

A variety of processes for producing VGCF are disclosed, including aprocess in which an organic compound such as benzene, serving as a rawmaterial, and an organic transition metal compound such as ferrocene,serving as a catalyst, are introduced into a high-temperature reactionfurnace together with a carrier gas, to thereby produce VGCF on asubstrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700);a process in which VGCF is produced in a dispersed state (JapanesePatent Application Laid-Open (kokai) No. 60-54998 (U.S. Pat. No.4,572,813)); and a process in which VGCF is grown on a reaction furnacewall by means of spraying onto the furnace wall droplets of a solutioncontaining a raw material and a metallic catalyst (Japanese Patent No.2778434).

The aforementioned processes have enabled production of carbon fiber ofrelatively small outer diameter and high aspect ratio which exhibitsexcellent electrical conductivity and heat conductivity and is suitableas a filler material. For example, carbon fiber having an outer diameterof about 10 to about 200 nm and an aspect ratio of about 10 to about 500has been mass-produced and used, for example, as an electricallyconductive or heat conductive filler material to be incorporated intoelectrically conductive resin, or as an additive to be incorporated intolead storage batteries.

A characteristic feature of a VGCF filament resides in its shape andcrystal structure. A VGCF filament has a multi-layered shell structurehaving a very thin central hollow portion, wherein a plurality of carbonhexagonal network layers are grown around the hollow portion so as toform annual rings.

A carbon nano-tube, which is a type of carbon fiber having a diametersmaller than that of VGCF, has been discovered in soot obtained byevaporating a carbon electrode through arc discharge in helium gas. Thecarbon nano-tube has a diameter of 1–30 nm, and has a structure similarto that of a VGCF filament; i.e., the tube has a hollow cylindricalstructure having a central hollow portion, wherein a plurality of carbonhexagonal network layers are grown around the hollow portion so as toform annual rings. However, the process for producing the nano-tubethrough arc discharge is not carried out in practice, since the processis not suitable for mass production.

Meanwhile, carbon fiber of high aspect ratio and high conductivity canbe produced through the vapor-growth process, and therefore variousimprovements to the carbon fiber have been made. For example, U.S. Pat.No. 4,663,230 and Japanese Patent Publication (kokoku) No. 3-64606(European Patent No. 205556) disclose a graphitic cylindrical carbonfibril having an outer diameter of about 3.5 to about 70 nm and anaspect ratio of at least 100. The carbon fibril has a structure suchthat a plurality of layers of ordered carbon atoms are continuouslydisposed concentrically around the longitudinal axis of the fibril, andthe C-axis of each of the layers is substantially perpendicular to thelongitudinal axis. The entirety of the fibril has a smooth surface, andincludes no thermal carbon overcoat deposited through thermaldecomposition. Japanese Patent Application Laid-Open (kokai) No.61-70014 discloses vapor grown carbon fiber having an outer diameter of10–500 nm and an aspect ratio of 2–30,000, the thermal decompositioncarbon layer of the carbon fiber having a thickness of 20% or less thediameter of the carbon fiber. However, detailed studies have not yetbeen performed on the branched hollow structure, compressed specificresistance, and heat conductivity of the aforementioned carbon fibers.

Carbon fiber has low contact resistance, and, as compared withconventional carbon black or similar material, exhibits excellentelectrical conductivity and heat conductivity, and has high strength,since, in carbon fiber, carbon structure is developed along alongitudinal direction of a fiber filament, and fiber filaments areentangled extensively with one another. Therefore, various attempts havebeen made to enhance such characteristics of carbon fiber. For example,Japanese Patent No. 2862578 (European Patent No.491728) discloses thatthe contact resistance of carbon fiber is reduced by incorporating, intoa resin composition, carbon fiber containing entangled fiber filaments.Japanese Patent No. 1327970 discloses branched VGCF in which fresh VGCFis grown on a VGCF substrate. Japanese Patent Application Laid-Open(kokai) No. 6-316816 discloses VGCF having gnarled depositions thereon.

The aforementioned attempts have been made in order to ensure contactbetween fine carbon fiber filaments in a composite material, by bringingthe filaments into contact with one another or by bonding the filamentswith one another in advance. In addition to such carbon fiber filaments,there has been a demand for a single carbon fiber filament of enhancedelectrical conductivity or heat conductivity.

DISCLOSURE OF THE INVENTION

The present inventors have improved the structure of VGCF, and haveobtained branched vapor-grown carbon fiber having a very small outerdiameter, each fiber filament having a hollow cylindrical structure suchthat a central hollow portion extends throughout the filament includinga branched portion thereof; i.e., branched vapor-grown carbon fiber ofvery small outer diameter exhibiting excellent electrical conductivityand heat conductivity. The branched vapor-grown carbon fiber has a verysmall outer diameter, each fiber filament having a hollow cylindricalstructure such that a central hollow portion extends throughout thefilament including a branched portion thereof, the carbon fiber havinghigh electrical conductivity and heat conductivity. When the carbonfiber is added to a material such as resin or rubber or to electrodes ofvarious batteries, the carbon fiber filaments are dispersed so as toform a network structure, to thereby enhance electrical conductivity andheat conductivity of such a material.

That is, the present invention provides a branched vapor-grown carbonfiber, a process for producing it, an electrically conductivetransparent composition and an electrically conductive transparentmaterial formed therefrom having the following constituent features.

-   1. Branched vapor-grown carbon fiber having an outer diameter of 0.5    μm or less and an aspect ratio of at least 10, each fiber filament    having a hollow cylindrical structure, characterized by having a    compressed specific resistance of 0.02 Ω·cm or less;-   2. Branched vapor-grown carbon fiber according to 1 above, which has    an outer diameter of 0.05–0.5 μm, a length of 1–100 μm, and an    aspect ratio of 10–2,000;-   3. Branched vapor-grown carbon fiber according to 1 above, which has    an outer diameter of 0.002–0.05 μm, a length of 0.5–50 μm, and an    aspect ratio of 10–25,000;-   4. Branched vapor-grown carbon fiber according to 2 or 3 above,    which has a compressed specific resistance of 0.018 Ω·cm or less,    each fiber filament having a structure such that a central hollow    portion extends throughout the filament including a branched portion    thereof;-   5. Branched vapor-grown carbon fiber according to 4 above, which    comprises, in an amount of at least 10 mass %, branched carbon    fiber, each fiber filament having a structure such that a central    hollow portion extends throughout the filament including a branched    portion thereof;-   6. Branched vapor-grown carbon fiber according to 1 above, which    further comprises boron;-   7. Branched vapor-grown carbon fiber according to 6 above, which    comprises boron in an amount of 0.01–5 mass %;-   8. Branched vapor-grown carbon fiber according to any one of 1 to 7    above, which has a heat conductivity of at least 100 kcal(mh° C.)⁻¹;-   9. Branched vapor-grown carbon fiber according to 8 above, which has    a heat conductivity of at least 100 kcal(mh° C.)⁻¹ when the fiber is    compressed so as to attain a bulk density of 0.8 g/cm³;-   10. A process for producing branched vapor grown carbon fiber    according to 1 above, by thermal decomposition of an organic    compound with a transition metal catalyst, characterized by spraying    droplets of organic compound containing 5–10 mass % of a transition    metal element or its compound on a heating furnace wall to allow    reaction to form carbon fiber filaments on the furnace wall, burning    the recovered filaments at 800–1,500° C. in a non-oxidative    atmosphere, and heating them at 2,000–3,000° C. to perform    graphitization treatment in a non-oxidative atmosphere;-   11. A process according to 10 above, wherein the heating for    graphitization treatment is performed after doping with boron or at    least one boron compound selected from the group consisting of boron    oxide, boron carbide, boric ester, boric acid or its salt, and    organic boron compounds as a crystallization promotion compound in    an amount of 0.1–5 mass % in terms of boron;-   12. An electrically conductive transparent composition comprising a    resin binder and carbon fiber incorporated into the binder,    characterized by having transparency and comprising vapor grown    carbon fiber having an outer diameter of 0.01–0.1 μm, an aspect    ratio of 10–15,000, and a compressed specific resistance of 0.02    Ω·cm or less;-   13. An electrically conductive transparent composition according to    12 above, wherein the carbon fiber is vapor grown carbon fiber    having an outer diameter of 0.05–0.1 μm or less, a length of 1–100    μm, and an aspect ratio of 10–2,000, each fiber filament having a    hollow cylindrical structure;-   14. An electrically conductive transparent composition according to    12 above, wherein the blending amount of vapor grown carbon fiber is    5–40 mass % of the total composition;-   15. An electrically conductive transparent composition according to    12 above, which has a surface resistivity of 10,000 Ω/□ or less;-   16. An electrically conductive transparent composition according to    12 above, which has a surface resistivity of 5–10,000 Ω/□, and a    transmittance of at least 60% when the composition is formed to have    a thickness of 0.5 μm;-   17. An electrically conductive transparent composition according to    12 or 13 above, wherein the carbon fiber is vapor grown carbon fiber    having an interlayer distance (d₀₀₂) of carbon crystal layers of    0.339 nm or less and a compressed specific resistance of 0.018 Ω·cm    or less;-   18. An electrically conductive transparent composition according to    13 above, wherein the branched vapor grown carbon fiber has a    compressed specific resistance of 0.018 Ω·cm or less, each fiber    filament thereof having a structure such that a central hollow    portion extends throughout the filament including a branched portion    thereof;-   19. An electrically conductive transparent composition according to    18 above, wherein the carbon fiber comprises, in an amount of at    least 10 mass %, branched vapor-grown carbon fiber, each fiber    filament having a structure in which a central hollow portion    extends throughout the filament including a branched portion    thereof;-   20. An electrically conductive transparent composition according to    12 or 13 above, wherein the vapor grown carbon fiber comprises boron    or a combination of boron and nitrogen in an amount of 0.01–3 mass    %;-   21. An electrically conductive transparent composition according to    12 or 13 above, wherein the vapor grown carbon fiber comprises    fluorine in an amount of 0.001–0.05 mass %;-   22. An electrically conductive transparent composition according to    12 or 13 above, wherein the vapor grown carbon fiber is coated with    20–70 mass % aluminum oxide;-   23. An electrically conductive transparent composition according to    12 or 13 above, which comprises carbon black together with the vapor    grown carbon fiber;-   24. An electrically conductive transparent material formed from an    electrically conductive transparent composition as recited in any    one of 12 through 23 above; and-   25. An electrically conductive transparent material according to 24    above, which assumes a form of coating, film produced through    spraying, film, or sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a photomicrograph of the branched vapor-grown carbon fiberof the present invention as obtained by use of a transmission electronmicroscope (magnification: ×100,000).

FIG. 2 shows a photomicrograph of a branched portion of the branchedvapor-grown carbon fiber of the present invention (magnification:×100,000).

FIG. 3 shows a photomicrograph of the conventional branched vapor-growncarbon fiber as obtained by use of a transmission electron microscope(magnification: ×100,000).

FIG. 4 is a schematic longitudinal cross-section showing a cell for themeasurement of compressed specific resistance of the carbon fiber of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

First of all, the branched vapor-grown carbon fiber of the presentinvention will be explained.

The present invention provides a branched carbon fiber produced throughthe vapor-growth process, which has an outer diameter of 0.5 μm or less,an aspect ratio of at least 10, and a compressed specific resistance of0.02 Ω·cm or less, each fiber filament having a hollow cylindricalstructure. Preferably, the branched vapor-grown carbon fiber has acompressed specific resistance of 0.018 Ω·cm or less, each fiberfilament having a structure such that a central hollow portion extendsthroughout the filament including a branched portion thereof.

As shown in photomicrographs of FIGS. 1 and 2 (magnification: ×100,000),in preferred branched vapor-grown carbon fiber of the present invention,each fiber filament has a structure such that a central hollow portionextends throughout the filament including a branched portion thereof. Asa result, the sheath-forming carbon layers of the carbon fiber assumeuninterrupted layers so that although the filaments of the carbon fiberhave a very small diameter, the carbon fiber exhibits excellentelectrical conductivity and heat conductivity. The electricalconductivity and heat conductivity of a conventional carbon fiber varywith the degree of contact or adhesion between the fiber filaments.Since branched portions of the conventional carbon fiber are bonded withone another so as to form nodules as shown in, for example, thephotomicrograph of FIG. 3 (magnification: ×100,000), the electricalconductivity and heat conductivity of the conventional carbon fiber arelower than those of the carbon fiber of the present invention.

As used herein, the term “hollow cylindrical structure” of the branchedvapor-grown carbon fiber refers to a structure such that a plurality ofcarbon layers form a sheath. The hollow cylindrical structureencompasses a structure such that sheath-forming carbon layers form anincomplete carbon sheet; a structure such that the carbon layers arepartially broken; and a structure such that the laminated two carbonlayers are formed into a single carbon layer. The cross section of thesheath does not necessarily assume a round shape, and may assume anelliptical shape or a polygonal shape. No particular limitation isimposed on the interlayer distance (d₀₀₂) of carbon crystal layers. Theinterlayer distance (d₀₀₂) of the carbon layers as measured throughX-ray diffraction is preferably 0.339 nm or less, more preferably 0.338nm or less. The thickness (Lc) of the carbon crystal layer in the c axisdirection is preferably 40 nm or less.

The branched vapor-grown carbon fiber of the present invention has avery small diameter; i.e., an outer diameter of 0.5 μm or less, and anaspect ratio of at least 10. Preferably, the carbon fiber has an outerdiameter of 0.05–0.5 μm and a length of 1–100 μm (i.e., an aspect ratioof 10–2,000); or an outer diameter of 0.002–0.05 μm and a length of0.5–50 μm (i.e., an aspect ratio of 10–25,000). When the outer diameterof the carbon fiber exceeds 0.5 μm, mixing of the carbon fiber in theresin is difficult, which is not preferable. In contrast, when the outerdiameter of the carbon fiber is less than 0.002 μm, the strength of thecarbon fiber is lowered, allowing the fiber to break easily, which isnot preferable.

Although carbon fiber having an outer diameter of 0.05–0.5 μm and alength of 1–100 μm can be produced through the process for producingbranched vapor-grown carbon fiber (Japanese Patent No. 2778434) in whichdroplets of a solution including a raw material and a metallic catalystare sprayed onto a reaction furnace wall, the carbon fiber of thepresent invention has an outer diameter smaller than that of the abovecarbon fiber by one digit; i.e., an outer diameter of 0.01–0.1 μm. Sucha very thin carbon fiber can be produced by utilizing the catalyticaction of a crystallization promotion element, preferably boron, etc.;i.e., by doping (adding a small amount of) this element to carboncrystals, during graphitization of deposited carbon fiber. The dopingamount of the element in terms of boron is suitably 0.01 to 5 mass %,preferably 0.1 to 3 mass %. When the amount of boron exceeds 5 mass %,doping with boron is difficult, whereas when the amount of boron is lessthan 0.01 mass %, the effect of boron is not satisfactory. When boron isincorporated into carbon crystals, the interlayer distance (d₀₀₂) ofcarbon layers is reduced, allowing crystallization to proceed.

The branched vapor-grown carbon fiber of the present invention has acompressed specific resistance when the fiber is compressed so as toattain a bulk density of 0.8 g/cm³ (hereinafter the resistance will besimply referred to as “compressed specific resistance”) of 0.02 Ω·cm orless, preferably 0.018 Ω·cm or less. As described below in Examples,carbon fiber including branched fiber, which is produced through theconventional vapor-growth process, has a compressed specific resistanceof about 0.021 Ω·cm. When such a conventional carbon fiber is mixed witha resin to thereby prepare a conductive paste, the volume resistance ofthe paste is on the order of 0.38–0.45 Ω·cm. In contrast, as shown inExamples, the carbon fiber including branched fiber of the presentinvention has an electrical conductivity higher than that of theconventional carbon fiber, and has a compressed specific resistance of0.005–0.018 Ω·cm.

The branched vapor-grown carbon fiber of the present invention has aheat conductivity of at least 100 kcal(mh° C.)⁻¹, or a heat conductivitywhen the fiber is compressed so as to attain a bulk density of 0.8 g/cm³of at least 100 kcal(mh° C.)⁻¹. Since the carbon fiber has a branchedshape and enhanced crystallinity, when the fiber is mixed with a resin,the heat conductivity of the resultant composite material can beenhanced. In order to obtain the effect of the branched vapor-growncarbon fiber, the carbon fiber is preferably incorporated into a resinin an amount of at least 10 mass %. Heat conductivity correlates toelectrical conductivity; i.e., when electrical conductivity is high,heat conductivity is also high.

The aforementioned branched vapor-grown carbon fiber of the presentinvention can be used, in a variety of fields, as a material for resinfiller for use in magnetic wave shielding materials and antistaticmaterials, conductive ink, a conductive paste, a transparent electrode,electrode additive, conductivity imparting agent for photoconductordrums, optical material, high-strength-structure material, and heatconductive material.

The process for producing the branched vapor-grown carbon fiber of thepresent invention will next be described.

Branched vapor grown carbon fiber of the present invention can beproduced according to the process for producing vapor-grown carbon fiber(Japanese Patent No. 2778434) in which droplets of a solution includinga raw material and a metallic catalyst are sprayed onto a reactionfurnace wall.

First, crude fine carbon fiber filaments are obtained by thermaldecomposition of an organic compound, in particular a hydrocarbon, byuse of an organic transition metal compound serving as a catalyst.

The organic transition metal compound as used herein includes organiccompounds that contain metals belonging to the Group IVa, Va, VIa, VIIaand VIII in the periodic table. Among them, those compounds such asferrocene and nickelocene are preferred.

In order to increase the content of branched carbon fiber, theconcentration of a metallic catalyst such as ferrocene, which is addedto a raw material, is preferably increased. Conventionally, theconcentration of the metallic catalyst is about 4 mass %, but in thepresent invention the concentration of the metallic catalyst ispreferably 5–10 mass %, more preferably about 7 mass %.

In addition, a sulfur compound may be used as a promoter. The form ofthe sulfur compound is not particularly limited as far as it isdissolved in an organic compound as a carbon source. The sulfur compoundthat can be used includes thiophene, various types of thiols, inorganicsulfur and so forth. The use amount thereof is suitably 0.01–10.0 mass%, preferably 0.03–5.0 mass %, more preferably 0.1–4.0 mass %.

The organic compound that can be used as a raw material for the carbonfiber includes organic compounds such as benzene, toluene, xylene,methanol, ethanol, naphthalene, phenanthrene, cyclopropane, cyclopentaneand cyclohexane; volatile oils; kerosene; or gases such as CO, naturalgas, methane, ethane, ethylene and acetylene, and mixtures thereof.Among then, aromatic compounds such as benzene, toluene and xylene areparticularly preferred.

Usually, hydrogen gas and other reducing gases are used as a carriergas. It is preferred that the carrier gas be preliminarily heated at500–1,300° C. The reason for heating is that both generation of acatalyst metal and supply of a carbon source through thermaldecomposition of the carbon compound can take place simultaneously sothat the reaction can complete instantaneously to obtain a finer carbonfiber. When the carrier gas is mixed with the raw material, the thermaldecomposition of the carbon compound as a raw material can barely occurif the temperature for heating the carrier gas is below 500° C. while ifsuch heating temperature exceeds 1,300° C., the carbon fiber grows inthe radial direction, so that the diameter tends to become larger.

The use amount of carrier gas is suitably 1–70 mol per mol of the carbonsource (organic compound). The diameter of the carbon fiber can becontrolled by varying the ratio of the carbon source to the carrier gas.

The raw material is prepared by dissolving a transition metal compoundand a sulfur compound as a promoter in an organic compound as a carbonsource.

There has been conventionally known a process for producing branchedcarbon fiber in which a raw material and a metallic catalyst aregasified, and fed to a reaction furnace. However, this conventionalprocess can barely generate branched carbon fiber. Accordingly, in thepresent invention it is preferred that a solution including an organiccompound raw material such as benzene and a metallic catalyst such asferrocene be sprayed and fed in the form of a liquid into the reactionfurnace or a portion of the carrier gas be used as a purge gas to gasifythe solution before it can be fed into the reaction furnace. In order toobtain carbon fiber having a smaller diameter, it is preferably that agas obtained by gasifying the solution is fed into the reaction furnace.When the solution is sprayed in the form of a liquid onto a reactionfurnace wall to thereby allow reaction to proceed, the concentration ofthe raw material and the metallic catalyst increases locally, and thusbranched carbon fiber is easily deposited. Through recovery andcrystallization of the thus-deposited carbon fiber, there can beproduced branched vapor-grown carbon fiber containing, in an amount ofat least 10 mass %, branched carbon fiber filaments having a structurein which central hollow portions extend throughout the filamentsincluding branched portions thereof.

As the reaction furnace, usually a vertical type electric furnace isused. The temperature of reaction furnace is 800–1,300° C., preferably1,000–1,300° C. By feeding the raw material solution and the carriergas, or the raw material gas obtained by gasifying the raw material andthe carrier gas to the reaction furnace the temperature of which hasbeen elevated to a predetermined temperature to allow them to react witheach other to obtain carbon fiber.

After the carbon fiber containing branched carbon fiber filamentsproduced in the reaction furnace is recovered, the carbon fiber isheated and fired at 800–1,500° C. in a non-oxidizing atmosphere such asargon gas, to thereby allow crystallization to proceed. Subsequently,the thus-crystallized carbon fiber is further heated at 2,000–3,000° C.in a non-oxidizing atmosphere, to thereby allow graphitization toproceed. During this graphitization, the crystallized carbon fiber isdoped with a crystallization promotion element (by addition of a smallamount of it), to thereby enhance crystallinity of the fiber. Thecrystallization promotion element is preferably boron. Since thegraphitized fine carbon fiber is covered with a dense basal plane (aplane of hexagonal network structure), preferably, carbon fiber of lowcrystallinity, which has been heated at 1,500° C. or lower is doped withboron. In this case, also carbon fiber of high crystallinity can beobtained since the carbon fiber of low crystallinity is heated to itsgraphitization temperature when it is doped with boron; i.e., when it issubjected to boronization.

The doping amount of boron is typically 5 mass % or less with respect tothe amount of carbon. When carbon fiber is doped with boron in an amountof 0.1–5 mass % in terms of boron, the crystallinity of the carbon fibercan be effectively enhanced. Therefore, elementary boron or a boroncompound (e.g., boron oxide (B₂O₃), boron carbide (B₄C), a boric ester,boric acid (H₃BO₃) or a salt thereof, or an organic boron compound) as acrystallization promotion compound is added to carbon fiber such thatthe boron content of the carbon fiber falls within the above range. Inconsideration of percent conversion, the boron compound may be added inan amount of 0.1–5 mass % in terms of boron with respect to the amountof carbon. It should be noted, however, that only requirement is thatboron be present when the fiber is crystallized through heat treatment.Boron may be evaporated during the course of high-temperature treatmentperformed after carbon fiber has been highly crystallized, to therebyreduce the boron content of the carbon fiber relative to the amount ofboron initially added to the fiber. However, such a reduction isacceptable only to such an extent that the amount of residual boron inthe fiber after the treatment is about 0.01 mass % or more.

The temperature required for introducing boron into carbon crystals orthe surface of carbon fiber is at least 2,000° C., preferably at least2,300° C. When the heating temperature is lower than 2,000° C.,introduction of boron becomes difficult, because of low reactivitybetween boron and carbon. In order to enhance crystallinity of carbonfiber, and to make the interlayer distance (d₀₀₂) of carbon crystallayers 0.338 nm or less, the heating temperature is preferablymaintained at 2,300° C. or higher. The heat treatment is carried out ina non-oxidizing atmosphere, preferably in an atmosphere of rare gas suchas argon. When the heat treatment is carried out for a very long periodof time, sintering of carbon fiber proceeds, resulting in a low yield.Therefore, after the temperature of the center portion of carbon fiberreaches the target temperature, the carbon fiber is maintained at thetarget temperature within about one hour.

Carbon fiber produced through the vapor-growth process has a very smallbulk density. Therefore, preferably, after the carbon fiber is uniformlymixed with boron or a boron compound, the resultant mixture is subjectedto shaping, granulation, or compression, and the resultant carbon fiberof high density is heated. When carbon fiber of high density issubjected to heat treatment, a portion of the fiber is sintered tobecome flocky. Therefore, after the flocky portion is pulverized, thecarbon fiber is used in a variety of materials.

Next, electrically conductive transparent composition of the presentinvention will be explained.

The electrically conductive transparent composition of the presentinvention contains a binder formed from a resin, particularly atransparent resin, and carbon fiber incorporated into the binder. Acharacteristic feature of the composition resides in that thecomposition contains vapor grown carbon fiber having an outer diameterof 0.01–0.1 μm, an aspect ratio of 10–15,000, and a compressed specificresistance of 0.02 Ω·cm or less, and that the composition has a surfaceresistivity of 10,000 Ω/□ or less. The composition of the presentinvention has both transparency and high electrical conductivity and isused as a transparent electrode for coating, film produced throughspraying, film, or sheet.

The carbon fiber used in the electrically conductive transparentcomposition of the present invention is produced through thevapor-growth process. As aforementioned, vapor grown carbon fiber (VGCF)is produced by thermally decomposing a raw material gas, such ashydrocarbon gas, in a vapor phase in the presence of a metalliccatalyst, and by growing the decomposition product into a fibrous shape.A variety of processes for producing VGCF are disclosed, including aprocess in which an organic compound such as benzene, serving as a rawmaterial, and an organic transition metal compound such as ferrocene,serving as a catalyst, are introduced into a high-temperature reactionfurnace together with a carrier gas, to thereby produce VGCF on asubstrate (Japanese Patent Application Laid-Open (kokai) No. 60-27700);a process in which VGCF is produced in a dispersed state (JapanesePatent Application Laid-Open (kokai) No. 60-54998); and a process inwhich VGCF is grown on a reaction furnace wall by means of spraying ontothe furnace wall droplets of a solution containing a raw material and ametallic catalyst (Japanese Patent No. 2778434). The aforementionedprocesses have enabled production of, for example, VGCF having an outerdiameter of about 0.01 to about 0.5 μm and an aspect ratio of about 10to about 500.

In the present invention, the carbon fiber used is vapor grown carbonfiber having an outer diameter of 0.01–0.1 μm and an aspect ratio of10–15,000. When carbon fiber having an outer diameter of more than 0.1μm is incorporated into a resin, the transparency of the resin isgreatly lowered. In contrast, when the outer diameter of carbon fiber isless than 0.01 μm, the strength of the carbon fiber is reduced, and thuswhen the carbon fiber is incorporated into a resin, the fiber is easilybroken. Meanwhile, when the aspect ratio of carbon fiber is more than15,000; i.e., when carbon fiber is very long, fiber filaments areexcessively entangled and as a result uniform dispersion of the carbonfiber in a resin becomes difficult.

Carbon fiber having an outer diameter of 0.05–0.5 μm and a length of1–100 μm can be produced through the process for producing vapor-growncarbon fiber (Japanese Patent No. 2778434) in which droplets of asolution including a raw material and a metallic catalyst are sprayedonto a reaction furnace wall. However, the electrically conductivetransparent composition of the present invention employs carbon fiberhaving an outer diameter of 0.01–0.1 μm. In order to obtain a very finecarbon fiber having further improved crystallinity, the deposited carbonfiber may be graphitized. In this case, utilizing the catalytic actionof a crystallization promotion element, e.g., boron or a combination ofboron and nitrogen; i.e., by doping carbon crystals or the surface ofcarbon fiber with such an element, graphitized carbon fiber can beobtained. The doping amount of such an element is 0.01–5 mass %,preferably 0.1–3 mass %, more preferably 0.2–2.0 mass %. When the amountof such an element exceeds 5 mass %, doping with the element isdifficult, whereas when the amount of the element is less than 0.01 mass%, the effect of the element is not satisfactory. When such an elementas boron is incorporated into carbon crystals, the interlayer distance(d₀₀₂) of carbon layers is reduced, allowing crystallization to proceed.As a result, there can be produced carbon fiber having, as compared withconventional carbon fiber, a small outer diameter, high electricalconductivity, and high dispersibility to a resin.

The vapor grown carbon fiber used in the electrically conductivetransparent composition of the present invention has a compressedspecific resistance of 0.02 Ω·cm or less, preferably 0.018 Ω·cm or less,more preferably 0.015 Ω·cm or less. Incidentally, the carbon fiberproduced through the conventional vapor-growth process has a compressedspecific resistance of about 0.021 Ω·cm. In contrast, the carbon fiberused in the present invention has an electrical conductivity higher thanthat of the conventional carbon fiber, and has a compressed specificresistance of, for example, 0.005–0.018 Ω·cm. When carbon fiber having acompressed specific resistance of more than 0.02 Ω·cm is used, obtaininga transparent composition having a surface resistivity of 10,000 Ω/□, orless is difficult.

The vapor grown carbon fiber preferably used in the electricallyconductive transparent composition of the present invention is branchedvapor grown branched carbon fiber as described above that contains alarge amount of branched carbon fiber, each fiber filament having astructure such that a central hollow portion extends throughout thefilament including a branched portion thereof. In such a vapor-grown,branched carbon fiber filament having a hollow cylindrical structure,sheath-forming carbon layers assume uninterrupted layers. Therefore,although having a very small diameter, the branched carbon fiberexhibits excellent electrical conductivity and heat conductivity. Theelectrical conductivity and heat conductivity of conventional carbonfiber vary with the degree of contact or adhesion between fiberfilaments. Since branched portions of the conventional carbon fiber arebonded with one another so as to form nodules, the electricalconductivity and heat conductivity of the conventional carbon fiber arelow as compared with the present branched carbon fiber, each fiberfilament having a structure such that a central hollow portion extendsthroughout the filament including a branched portion thereof.

The vapor grown carbon fiber used in the electrically conductivetransparent composition of the present invention may be treated withfluorine so as to contain 0.001–0.05 mass % fluorine. The fluorinetreatment is performed, for example, by performing contact treatment at0–200° C. in the presence of a fluorine containing gas (F₂, HF, etc) orby plasma treatment with a fluorinated lower hydrocarbon such as CF₄(for example, Japanese Patent Application Laid-open (Kokai) No.8-31404). When the carbon fiber is treated with fluorine, the repellencyof the surface of the carbon fiber is enhanced. As a result, carbonfiber filaments are not easily flocculated, and dispersibility of thecarbon fiber can be enhanced. When the fluorine content is less than0.001 mass %, the effect of fluorine treatment is unsatisfactory,whereas when the fluorine content exceeds 0.05 mass %, carbon crystalplanes are broken, and the surface of the carbon fiber becomes rough.

The vapor grown carbon fiber used in the present invention may betreated with an aluminum compound (e.g., alumina gel, aluminum chloride,aluminum sulfate, aluminum nitrate, aluminum silicate, an aluminate, analuminic ester, or aluminum hydroxide), preferably with alumina gel,aluminum silicate, an aluminate, or aluminum hydroxide, and may becoated with 20–70 mass % aluminum oxide. For example, an alumina filmmay be formed on the surface of fiber by activating the vapor growncarbon fiber with carbon dioxide to form activated carbon fiber, dippingit in about 10% sulfuric acid and washing with water, adding the fiberin an aluminum compound solution.

When the carbon fiber is subjected to such surface treatment, thehydrophilicity of the surface of the carbon fiber can be enhanced. As aresult, adhesion between the carbon fiber and a resin is enhanced, andthe dispersibility of the carbon fiber is enhanced. When the coatingamount is less than 20 mass %, the effect of coating is unsatisfactory,whereas when the coating amount exceeds 70 mass %, adhesion betweencarbon fiber filaments is increased.

In the electrically conductive transparent composition of presentinvention, a suitable incorporation amount of vapor grown carbon fiberis appropriately 5–40 mass %, preferably 5–20 mass %, on the basis ofthe entirety of the composition. When the incorporation amount fallswithin the above range, the composition has high transparency and highelectrical conductivity. Specifically, the composition has a surfaceresistivity of 10,000 Ω/□ or less, and can attain a transmittance of 70%or more when the composition is formed to have a thickness of 0.5 μm. Inthis connection, conventional electrically conductive coatings to whichcarbon black as a sole carbonaceous material is incorporated in anamount nearly equal to that of the corresponding carbonaceous materialincorporated into the present composition have a transmittance of 30% orless; i.e., light-penetrability of the coatings is very low; whereas,when the amount of carbon black incorporated into such conventionalcoatings is reduced such that the light-penetrability of the coatings ismaintained at a level comparable to that of a coating formed from thepresent composition, the surface resistivity of the conventionalcoatings becomes 20,000 Ω/□ or more; i.e., the electrical conductivityof the coatings is greatly reduced.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be illustrated by way ofexamples and comparative examples. However, the present invention shouldnot be construed as being limited by the following description.

In the following examples, branched fiber content (area %), boroncontent, bulk density (tapping density) (g/cm³), compressed specificresistance (Ω·cm), specific resistance of paste (Ω·cm), surfaceresistivity of coating (Ω/□), transmittance (%) were measured by thefollowing processes.

1) Branched Fiber Content (Mass %):

In a photograph showing cross section of carbon fiber by use oftransmission electron microscope (TEM), a ratio of the cross sectionalarea of branched carbon fiber filaments to the total cross sectionalarea of carbon fiber filaments was obtained and assuming that thespecific density is 1, the ratio was defined as mass %.

2) Boron Content:

Powder sample of carbon fiber to which calcium carbonate was added wasincinerated at 800° C. in an oxygen flow. Then, after adding calciumcarbonate, the obtained ash was heat-molten and the melt was dissolvedin water. The resultant aqueous solution was subjected to quantitativeanalysis by use of inductively coupled plasma (ICP) emission spectralanalysis.

3) Bulk Density (Tapping Density) (g/cm³):

A predetermined amount (6.0 g) of sample was weighed and placed in a15-mmφ cell for measurement, which was set in a tapping apparatus. At afalling height of 45 mm and a tapping speed of 2 second/time, the samplewas freely fallen 400 times. Thereafter, the volume of the sample wasmeasured. From the relationship between the volume and mass, the bulkdensity of the sample was calculated.

4) Compressed Specific Resistance (Ω·cm):

Sample to be measured was placed in a resin cell 4 as shown in FIG. 4and pressed by compression rods 2 from above and below and current wasapplied at a constant pressure. Then the voltage between terminals forthe measurement of voltage placed at a midpoint of the sample was readand specific resistance was calculated from the cross sectional area ofthe vessel and the distance between the voltage terminals. The specificresistance varied depending on the pressing conditions and it showed ahigher resistance at a lower pressure whereas as the pressure is furtherincreased above a certain pressure, it showed a substantially constantvalue regardless of the pressing conditions. In the present invention,the value obtained when the sample was compacted to a bulk density of0.8 g/cm³ by the following operation was defined as volume specificresistance (compressed specific resistance).

That is, a predetermined amount of sample was placed in a cell 4 for themeasurement of compressed specific resistance made of a resin having aplanar area of 1×4 cm² and a depth of 10 cm, provided with a copperplate current terminal 3 for applying current to an object 5 to bemeasured and with a voltage measuring terminals 1 at a midpoint, and thesample was increasingly compressed by the compression rod 2 from aboveand while measuring the compression a current of 0.1 A was appliedthereto. When the bulk density of 0.8 g/cm³ was reached, the voltage (E)V between the two terminals 1 for the measurement of voltage at adistance of 2.0 cm therebetween inserted through the bottom of thevessel was read. Specific resistance (R) (Ω·cm) was calculated accordingto the following formula.R(Ω·cm)=(E/0.1)×D(cm ²)/2(cm)

In the above formula, D represents a cross sectional area (depth×width)of powder in the direction of current=10d.

5) Surface Resistivity of Coating Film (Ω/□):

A coating film was prepared and measured according to 4-terminal methodin compliance with “JIS K7194” by use of Lotest Hp MCP-T410 manufacturedby Mitsubishi Chemical, Inc.

6) Specific Resistance of Paste (Ω·cm):

A paste film sample having a film thickness of 25 μm was prepared by useof a doctor blade and the surface resistivity of the sample was measuredaccording to 5) above. The obtained value was divided by the filmthickness to obtain specific resistance of the paste.

7) Transmittance (%):

This was measured by integrating-sphere light transmittance method incompliance with “JIS K7105” by use of NDH-1001 DP manufactured by NipponDenshoku Industries Co., Ltd.

EXAMPLE 1

In accordance with the description in Japanese Patent No. 2778434, vaporgrown carbon fiber was produced through a production process in whichferrocene (7 mass %) was dissolved in benzene, and droplets of theresultant solution were sprayed onto a furnace wall, to thereby causethermal decomposition of the solution. The thus-produced carbon fiberwas heated at 1,200° C. in an argon atmosphere, and further heated at2,800° C. in an argon atmosphere. After heat treatment was completed,the resultant flocky carbon fiber was pulverized, to thereby yield vaporgrown carbon fiber having an outer diameter of 0.1–0.2 μm, a length of10–20 μm, and an aspect ratio of 50–200. Through observation by use of atransmission electron microscope (TEM), the carbon fiber was found tocontain branched carbon fiber in an amount of 22 mass %. The bulkdensity (tapping density) of the carbon fiber was 0.035 g/cm³. After thecarbon fiber was compressed so as to attain a bulk density of 0.8 g/cm³,the resultant carbon fiber had a compressed specific resistance (powderresistance) of 0.018 Ω·cm. The carbon fiber (40 mass %) was mixed withpolyurethane, to thereby prepare a paste. The specific resistance of thepaste was 0.25 Ω·cm. The results are shown in Table 1. FIG. 1 shows aphotomicrograph (magnification: ×100,000) of a branched portion of thecarbon fiber.

EXAMPLE 2

Boron carbide (B₄C) powder (4 mass %) was added to vapor grown carbonfiber containing branched carbon fiber produced in a manner similar tothat of Example 1, and uniform mixing was carried out. The resultantmixture was placed in a graphitic crucible, compressed, and then heatedat 2,700° C. in an argon atmosphere for 60 minutes. The resultantproduct was pulverized, to thereby yield boron-containing vapor growncarbon fiber containing branched carbon fiber. The boron content of thecarbon fiber was 1.8 mass %. In a manner similar to that of Example 1,the bulk density and compressed powder specific resistance of the carbonfiber, and the specific resistance of a resin paste containing the fiberwere measured. The bulk density (tapping density) was 0.036 g/cm³; thecompressed powder specific resistance was 0.005 Ω·cm; and the specificresistance of the resin paste was 0.08 Ω·cm. The results are shown inTable 1. Further, FIG. 2 shows a photomicrograph (magnification:×100,000) of a branched portion of the carbon fiber. The state wasobserved in which a central hollow portion extends throughout thefilament including a branched portion thereof.

EXAMPLES 3 AND 4

Boron carbide (B₄C) powder was added to vapor grown carbon fibercontaining branched carbon fiber produced in a manner similar to that ofExample 1, and uniform mixing was carried out. The resultant mixture wasplaced in a graphitic crucible, compressed, and then heated at2,800–2,900° C. in an argon atmosphere for 60 minutes, to thereby yieldboron-containing vapor grown carbon fiber containing branched carbonfiber. In Example 3, the boron content of the carbon fiber was 0.5 mass%, and in Example 4, the boron content of the carbon fiber was 0.2 mass%. In a manner similar to that of Example 1, the bulk density andcompressed powder specific resistance of the carbon fiber, and thespecific resistance of a resin paste containing the fiber were measured.The results are shown in Table 1 together with the boron content.

COMPARATIVE EXAMPLE 1

Vapor grown carbon fiber (outer diameter: 0.1–0.2 μm, length: 10–20 μm)was produced through, instead of the production process described inExample 1, a conventional production process in which a raw material wasgasified and then fed into a furnace. Through observation by use of aTEM, the thus-produced carbon fiber was found to contain only a smallamount of branched carbon fiber. In a manner similar to that of Example1, the bulk density and compressed powder specific resistance of thecarbon fiber, and the specific resistance of a resin paste containingthe fiber were measured. The results are shown in Table 1.

COMPARATIVE EXAMPLE 2

The procedure of Example 1 was repeated, except that the amount offerrocene added was reduced to 2 mass %, to thereby produce vapor growncarbon fiber (outer diameter: 0.1–0.2 μm, length: 5–10 μm). Throughobservation by use of a TEM, the carbon fiber was found to containbranched carbon fiber in an amount of 5 mass %. In a manner similar tothat of Example 1, the bulk density and compressed powder specificresistance of the carbon fiber, and the specific resistance of a resinpaste containing the fiber were measured. The results are shown inTable 1. FIG. 3 shows a photomicrograph of a branched portion of thecarbon fiber.

TABLE 1 Branched Compressed Specific Carbon fiber (μm) carbon Bulkspecific resistance Outer fiber Boron density resistance of pastediameter Length content content (g/cm³) (Ω · cm) (Ω · cm) Example 10.1–0.2 10–20 22 0 0.035 0.018 0.25 2 0.1–0.2 10–20 22 1.8 0.036 0.0050.08 3 0.1–0.2 10–20 22 0.5 0.035 0.005 0.09 4 0.1–0.2 10–20 22 0.20.036 0.005 0.08 Comparative 1 0.1–0.2 10–20 0 0 0.035 0.022 0.45Example 2 0.1–0.2  5–10 5 0 0.035 0.021 0.38 Note: each content isrepresented by mass %.

As shown in Table 1, the branched vapor-grown carbon fiber of thepresent invention (Examples 1 through 4) has a compressed powderspecific resistance of 0.02 Ω·cm or less, which is lower than that ofthe conventional vapor grown carbon fiber (Comparative Examples 1 and2). Therefore, the specific resistance of a resin paste containing thepresent branched vapor-grown carbon fiber is low; i.e., 0.3 Ω·cm orless. In contrast, the conventional vapor grown carbon fiber(Comparative Examples 1 and 2) has a compressed powder specificresistance of higher than 0.02 Ω·cm. Since the present branchedvapor-grown carbon fiber containing boron has high crystallinity, itscompressed powder specific resistance is further reduced.

Furthermore, as shown in the photomicrographs (magnification: ×100,000)in FIGS. 1 and 2 as obtained by use of a transmission electronmicroscope, in the present branched vapor-grown carbon fiber of thepresent invention, an individual fiber filament does not have nodules ona branched portion thereof, and a central hollow portion extendsthroughout the filament including the branched portion.

EXAMPLE 5

In the same manner as in Example 1, vapor grown carbon fiber having anaverage outer diameter of 0.04 μm, an aspect ratio of about 40, and acompressed specific resistance of 0.015 Ω·cm was obtained. Fromobservations by use of a transmission electron microscope (TEM), it wasconfirmed that the carbon fiber contained 15 mass % of branched carbonfiber filaments.

0.5 mass part of the carbon fiber was added to a resin solutioncontaining a polyester resin (4.5 mass parts) and methyl ethyl ketone(MEK) (95 mass parts), and the carbon fiber was dispersed in thesolution by use of a paint shaker, to thereby yield an electricallyconductive transparent composition. The composition was applied onto aglass plate by use of a spin-coater so as to attain a film thickness of0.1 μm, and then dried at 150° C. for 1.5 hours. The transmittance at600 nm and surface resistivity of the resultant coating were measured.The surface resistivity and transmittance of the coating were 2,000 Ω/□and 80%, respectively.

EXAMPLE 6

In a manner similar to that of Example 5, a coating was formed by use ofthe same vapor grown carbon fiber as in Example 5; i.e., vapor growncarbon fiber having an average outer diameter of 0.04 μm, an aspectratio of about 40, and a compressed specific resistance of 0.015 Ω·cm(0.25 mass parts) and carbon black (Ketjen Black EC, product of AKZO)(0.25 mass parts). The surface resistivity and transmittance of thecoating were 1,500 Ω/□ and 75%, respectively.

EXAMPLE 7

The same vapor grown carbon fiber as in Example 5; i.e., vapor growncarbon fiber having an average outer diameter of 0.04 μm and an aspectratio of about 40 was mixed with 4 mass parts of boron carbide (B₄C),and the resultant mixture was subjected to heat treatment at 2,800° C.in an atmosphere of inert gas. After heat treatment was completed, theboron content of the carbon fiber was 1.8 mass %, the compressedspecific resistance of the carbon fiber was 0.008 Ω·cm, and theinterlayer distance d₀₀₂ was 0.3375 nm. In a manner similar to that ofExample 5, a coating was formed by use of 0.5 mass part of the resultantvapor grown carbon fiber. The surface resistivity and transmittance ofthe coating were 1,500 Ω/□ and 80%, respectively.

EXAMPLE 8

The vapor grown carbon fiber obtained in the same manner as in Example1; i.e., vapor grown carbon fiber having an outer diameter of 0.08 μm,an aspect ratio of about 40, and a compressed specific resistance of0.015 Ω·cm was treated at 35° C. in a fluorine (F₂) atmosphere. In amanner similar to that of Example 5, a coating was formed by use of theresultant vapor grown carbon fiber. The surface resistivity andtransmittance of the coating were 2,000 Ω/□ and 90%, respectively.

EXAMPLE 9

Vapor grown carbon fiber having an outer diameter of 0.08 μm, an aspectratio of about 40, and a compressed specific resistance of 0.018 Ω·cmwas activated with carbon dioxide gas, to thereby yield activated carbonfiber having a specific surface area of 2,000 m²/g. The carbon fiber wasimmersed in 10% sulfuric acid for one hour, and then washed with water.Subsequently, the resultant carbon fiber was added to a sodium aluminatesolution, to thereby form an alumina film (25 mass %) on the surface ofthe carbon fiber. In a manner similar to that of Example 5, a coatingwas formed by use of the resultant vapor grown carbon fiber. The surfaceresistivity and transmittance of the coating were 4,000 Ω/□ and 95%,respectively.

COMPARATIVE EXAMPLE 3

In a manner similar to that of Example 5, a coating was formed by use ofthe vapor grown carbon fiber obtained in the same manner as in Example1; i.e., vapor grown carbon fiber having an outer diameter of 0.5 μm, anaspect ratio of about 40, and a compressed specific resistance of 0.022Ω·cm. The surface resistivity and transmittance of the coating were2,500 Ω/□ and 35%, respectively.

COMPARATIVE EXAMPLE 4

In a manner similar to that of Example 5, a coating was formed by use ofcarbon black (Ketjen Black EC, product of AKZO) having a BET specificsurface area of 1,270 m²/g. The surface resistivity and transmittance ofthe coating were 3,000 Ω/□ and 10%, respectively.

INDUSTRIAL APPLICABILITY

The vapor grown carbon fiber containing branched carbon fiber of thepresent invention has a very small outer diameter, each fiber filamenthaving a hollow cylindrical structure in which a central hollow portionextends throughout the filament including a branched portion thereof,which carbon fiber has high electrical conductivity and heatconductivity. Therefore, when the carbon fiber is added to a materialsuch as resin or rubber or to electrodes of various batteries, thecarbon fiber filaments are dispersed so as to form a network structure,to thereby enhance electrical conductivity and heat conductivity of sucha material. In addition, since the present carbon fiber has a diametersmaller than that of conventional carbon fiber, even when the presentcarbon fiber is incorporated into a resin in a relatively large amount,transparency inherent to the resin can be maintained, and a transparentcoating, film, or sheet of high electrical conductivity can be formedfrom the resin.

The electrically conductive composition of the present invention doesnot lose transparency inherent to a resin and exhibits excellentelectrical conductivity. In general, an electrically conductivecomposition containing carbon powder or conventional carbon fiber haslow transparency. In contrast, the electrically conductive compositionof the present invention has both high electrical conductivity and hightransparency, since transparency of the resin is barely lowered evenwhen the amount of carbon fiber incorporated.

1. Branched vapor-grown carbon fiber which has a hollow cylindricalstructure and a compressed specific resistance of 0.018 Ω·cm or less,each fiber filament having a structure such that a central hollowportion extends throughout the filament including a branched portionthereof.
 2. Branched vapor-grown carbon fiber as claimed in claim 1,which has an outer diameter of 0.05–0.5 μm, a length of 1–100 μm, and anaspect ratio of 10–2,000.
 3. Branched vapor-grown carbon fiber asclaimed in claim 1, which has an outer diameter of 0.002–0.05 μm, alength of 0.5–50 μm, and an aspect ratio of 10–25,000.
 4. Branchedvapor-grown carbon fiber as claimed in claim 1, which comprises, in anamount of at least 10 mass %, branched carbon fiber, each fiber filamenthaving a structure such that a central hollow portion extends throughoutthe filament including a branched portion thereof.
 5. Branchedvapor-grown carbon fiber as claimed in claim 1, which further comprisesboron.
 6. Branched vapor-grown carbon fiber as claimed in claim 5, whichcomprises boron in an amount of 0.01–5 mass %.
 7. Branched vapor-growncarbon fiber as claimed in claim 1, which has a heat conductivity of atleast 100 kcal(mh° C.)⁻¹.
 8. Branched vapor-grown carbon fiber asclaimed in claim 7, which has a heat conductivity of at least 100 kcal(mh° C.)⁻¹ when the fiber is compressed so as to attain a bulk densityof 0.8 g/cm³.
 9. A process for producing branched vapor grown carbonfiber as claimed in claim 1 by thermal decomposition of an organiccompound with a transition metal catalyst, characterized by sprayingdroplets of organic compound containing 5–10 mass % of a transitionmetal element or its compound on a heating furnace wall to allowreaction to form carbon fiber filaments on the furnace wall, burning therecovered filaments at 8001,500° C. in a non-oxidative atmosphere, andheating them at 2,000–3,000° C. to perform graphitization treatment in anon-oxidative atmosphere.
 10. The process as claimed in claim 9, whereinthe heating for graphitization treatment is performed after doping withboron or at least one boron compound selected from the group consistingof boron oxide, boron carbide, boric ester, boric acid or its salt, andorganic boron compounds as a crystallization promotion compound in anamount of 0.1–5 mass % in terms of boron.
 11. An electrically conductivetransparent composition comprising a resin binder and carbon fiberincorporated into the binder, characterized by having transparency andcomprising branched vapor grown carbon fiber having an outer diameter of0.01–0.1 μm, an aspect ratio of 10–15,000, and a compressed specificresistance of 0.02 Ω·cm or less, wherein the blending amount of thevapor grown carbon fiber is from 5 to 40 mass % of the totalcomposition.
 12. The electrically conductive transparent composition asclaimed in claim 11, wherein the carbon fiber is vapor grown carbonfiber having an outer diameter of 0.05–0.1 μm or less, a length of 1–100μm, and an aspect ratio of 10–2,000, each fiber filament having a hollowcylindrical structure.
 13. The electrically conductive transparentcomposition as claimed in claim 11, which has a surface resistivity of10,000 Ω/□ or less.
 14. The electrically conductive transparentcomposition as claimed in claim 11, which has a surface resistivity of5–10,000 Ω/□, and a transmittance of at least 60% when the compositionis formed to have a thickness of 0.5 μm.
 15. The electrically conductivetransparent composition as claimed in claim 11, wherein the carbon fiberis vapor grown carbon fiber having an interlayer distance (d₀₀₂) ofcarbon crystal layers of 0.339 nm or less and a compressed specificresistance of 0.018 Ω·cm or less.
 16. The electrically conductivetransparent composition as claimed in claim 11, wherein the branchedvapor grown carbon fiber has a compressed specific resistance of 0.018Ω·cm or less, each fiber filament thereof having a structure such that acentral hollow portion extends throughout the filament including abranched portion thereof.
 17. The electrically conductive transparentcomposition as 10 claimed in claim 16, wherein the carbon fibercomprises, in an amount of at least 10 mass %, branched vapor-growncarbon fiber, each fiber filament having a structure in which a centralhollow portion extends throughout the filament including a branchedportion thereof.
 18. The electrically conductive transparent compositionas claimed in claim 11, wherein the vapor grown carbon fiber comprisesboron or a combination of boron and nitrogen in an amount of 0.0 1–3mass %.
 19. The electrically conductive transparent composition asclaimed in claim 11, wherein the vapor grown carbon fiber comprisesfluorine in an amount of 0.001–0.05 mass %.
 20. The electricallyconductive transparent composition as claimed in claim 11, wherein thevapor grown carbon fiber is coated with 20–70 mass % aluminum oxide. 21.The electrically conductive transparent composition as claimed in claim11, which comprises carbon black together with the vapor grown carbonfiber.
 22. An electrically conductive transparent material formed froman electrically conductive transparent composition according to claim11.
 23. The electrically conductive transparent material as claimed inclaim 22, which assumes a form of coating, film produced throughspraying, film, or sheet.